1. Field of the Invention
The present invention relates to methods for sequentially measuring impedance of a fuel cell, measurement devices, and fuel cell systems.
2. Description of the Related Art
Recently, fuel cell electric vehicles (FCEV) have drawn attention to promote environmental programs and to decrease fossil fuel consumption.
The fuel cell electric vehicle (hereinafter, referred to as a “vehicle” or “fuel cell vehicle”) includes a fuel cell whose impedance characteristic is a critical element for vehicle driving performance and should be sequentially determined.
The fuel cell, however, has different impedance values because of different power generation conditions (e.g., a temperature, pressure, relative humidity) and/or driving conditions (e.g., a stoppage, start, high speed driving, high altitude driving, hill climbing). In addition, even under constant power generation conditions and/or driving conditions, the impedance exhibits different values depending on different frequencies. Fuel cell's electrolyte membrane resistance and other parameters are measured at a high-frequency range. How easily a chemical reaction proceeds and other aspects are observed at a medium-frequency range. A phenomenon occurring during diffusion of fuel and/or oxygen, what is called a concentration overvoltage, is observed at a low-frequency range. Thus, when flooding (retained-water-mediated blockage of a gas passage) occurs, the concentration overvoltage increases. In view of the above, a reaction in a fuel cell is a multi-scale reaction regarding time and frequency aspects. Its impedance has different frequency characteristics, which should be determined as a frequency characteristic at a predetermined frequency range. Because of this, a simultaneous measurement of the impedance is desirably performed at a broad frequency range.
Examples of a known conventional method for measuring impedance of a fuel cell include a FRA (Frequency Response Analyzer) method including: applying a sinusoidal wave at a predetermined frequency for measurement; and repeatedly conducting the measurement while changing the frequency (Patent Literatures 1 to 3).
In addition, examples of the impedance measurement other than the FRA method include a method for using a FFT (Fast Fourier Transform) after inputting an artificial white signal. For example, Non-Patent Documents 1 to 3 employ a technique for identification using a synthesized signal containing a sinusoidal wave.